1. Field of the Invention
The present invention relates to a sheet thickness measuring apparatus for measuring the thickness of various sheets in an image forming apparatus, a sheet conveying apparatus or a sheet containing apparatus.
2. Related Background Art
Generally, in image forming apparatuses in which various sheets are handled, various obstacles or drawbacks often occur due to the deviation of the properties of sheets. In particular, many obstacles have a close relation to a thickness of the sheet, and, thus, various techniques for measuring the thickness of the sheet in an image forming apparatus have been proposed.
In one typical measuring technique, the photo-permeability of a sheet is measured by using an optical sensor of a permeable type and the thickness of the sheet is estimated on the basis of the measured result. In another measuring technique, a sheet is pinched between a pair of rollers, and the thickness of the sheet is measured by detecting the relative displacement of the rollers in a thickness direction of the sheet caused by the pinched sheet.
However, these conventional techniques have the following drawbacks.
In the above technique in which the sheet thickness is measured by using the optical sensor of a permeable type, the optical sensor is influenced by the color tone and tissue of the sheet, thereby causing a great error in the measurement of the thickness. On the other hand, in the above technique in which the sheet thickness is measured by using the rollers, since it is difficult to stabilize the mechanical movements of the rollers, setting aside the question of theory, it is difficult to put the technique to practical use. In other conventional techniques, many problems arise regarding the cost and stability.
Incidentally, although an electrostatic thickness measuring method is also known as a technique capable of measuring a thickness of a sheet with ease and with high accuracy, such method has not been applied to an image forming apparatus for the following reason. That is to say, although a paper sheet and a plastic film can be used in the image forming apparatus, particularly, since there are many kinds of paper sheets and the paper sheets are apt to be influenced by the environmental condition, the property value greatly differs from paper sheet to paper sheet. For example, the dielectric constant of the sheet is varied from paper sheet to paper sheet by about 50% at the maximum, and the resistivity of the sheet is varied by one figure depending upon the kind of paper sheet and by six figures depending upon the environment.